Unraveling Spatially Heterogeneous Ultrafast Carrier Dynamics of Single-Layer WSe2 by Femtosecond Time-Resolved Photoemission Electron Microscopy,Nano Letters

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Unraveling Spatially Heterogeneous Ultrafast Carrier Dynamics of Single-Layer WSe2 by Femtosecond Time-Resolved Photoemission Electron Microscopy
Nano Letters ( IF 9.6 ) Pub Date : 2018-07-03 00:00:00 , DOI: 10.1021/acs.nanolett.8b02103
Lin Wang ₁ , Ce Xu ₂ , Ming-Yang Li ₃ , Lain-Jong Li ₃ , Zhi-Heng Loh _{1,

2,

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Affiliation

Studies of the ultrafast carrier dynamics of transition metal dichalcogenides have employed spatially averaged measurements, which obfuscate the rich variety of dynamics that originate from the structural heterogeneity of these materials. Here, we employ femtosecond time-resolved photoemission electron microscopy (TR-PEEM) with sub-80 nm spatial resolution to image the ultrafast subpicosecond to picosecond carrier dynamics of monolayer tungsten diselenide (WSe₂). The dynamics observed following 2.41 eV pump and 3.61 eV probe occurs on two distinct time scales. The 0.1 ps process is assigned to electron cooling via intervalley scattering, whereas the picosecond dynamics is attributed to exciton–exciton annihilation. The 70 fs decay dynamics observed at negative time delay reflects electronic relaxation from the Γ point. Analysis of the TR-PEEM data furnishes the spatial distributions of the various time constants within a single WSe₂ flake. The spatial heterogeneity of the lifetime maps is consistent with increased disorder along the edges of the flake and the presence of nanoscale charge puddles in the interior. Our results indicate the need to go beyond spatially averaged time-resolved measurements to understand the influence of structural heterogeneities on the elementary carrier dynamics of two-dimensional materials.

中文翻译：

飞秒时间分辨光发射电子显微镜揭示单层WSe ₂的空间异质超快载流子动力学

对过渡金属二卤化物超快载流子动力学的研究已采用空间平均测量，这混淆了源于这些材料结构异质性的丰富的动力学变化。在这里，我们采用具有小于80 nm空间分辨率的飞秒时间分辨光电子显微镜（TR-PEEM）来成像单层二硒化钨（WSe _2）的超快皮秒至皮秒载流子动力学）。在2.41 eV泵和3.61 eV探头之后观察到的动力学发生在两个不同的时间尺度上。通过皮秒散射将0.1 ps的过程分配给电子冷却，而皮秒动力学归因于激子-激子an灭。在负时延处观察到的70 fs衰减动力学反映了从Γ点开始的电子弛豫。对TR-PEEM数据的分析提供了单个WSe _2中各种时间常数的空间分布薄片。寿命图的空间异质性与薄片边缘的无序性增加以及内部存在纳米级电荷坑有关。我们的结果表明，需要超越空间平均时间分辨测量来了解结构异质性对二维材料基本载流子动力学的影响。

更新日期：2018-07-03

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